|Working Group I: The Scientific Basis|
|Other reports in this collection|
14.2.4 The Global Carbon Cycle
From measurements of air trapped in ice cores and from direct measurements
of the atmosphere, we know that in the past 200 years the abundance of CO2 in
the atmosphere has increased by over 30% (i.e., from a concentration of 280
ppm by volume (pppmv) in 1700 to nearly 370 ppmv in 2000). We also know that
the concentration was relatively constant (roughly within ±10 ppmv of
275) for more than 1,000 years prior to the human-induced rapid increase in
atmospheric CO2 (see Chapter 3, Figures 3.2a and 3.2b).
Looking further back in time, we find an extraordinarily regular record of
change. The Vostok core (Figure 3.2d) captures a
remarkable and intriguing signal of the periodicity of inter-glacial and glacial
climate periods in step with the transfer of significant pools of carbon from
the land (most likely through the atmosphere) to the ocean and then the recovery
of terrestrial carbon back from the ocean. The repeated pattern of a 100 to
120 ppmv decline in atmospheric CO2 from an inter-glacial value of 280 to 300
ppmv to a 180 ppmv floor and then the rapid recovery as the planet exits glaciation
suggests a tightly governed control system. There is a similar methane (CH4)
cycle between 320 to 350 ppbv (parts per billion by volume) and 650 to 770 ppbv.
What begs explanation is not just the linked periodicity of carbon and glaciation,
but also the apparent consistent limits on the cycles over the period. See Chapter
3, Box 3.4.
Today’s atmosphere, imprinted with the fossil fuel CO2 signal, stands
at nearly 90 to 70 ppmv above the previous inter-glacial maximum of 280 to 300
ppmv. The current methane value is even further (percentage-wise) from its previous
inter-glacial high values. In essence, carbon has been moved from a relatively
immobile pool (in fossil fuel reserves) in the slow carbon cycle to the relatively
mobile pool (the atmosphere) in the fast carbon cycle, and the ocean, terrestrial
vegetation and soils have yet to equilibrate with this “rapidly” changing
concentration of CO2 in the atmosphere.
Given this remarkable and unprecedented history one cannot help but wonder
about the characteristics of the carbon cycle in the future (Chapter
3). To understand better the global carbon cycle, two themes are clear:
(1) there is a need for global observations that can contribute significantly
to determining the sources and sinks of carbon and (2) there is a need for fundamental
work on critical biological processes and their interaction with the physical
system. Two observational needs must be highlighted:
We note that the Subsidiary Body for Scientific and Technological Advice (SBSTA)
of the United Nations Framework Convention on Climate Change (UNFCCC) recognised
the importance of an Integrated Global Observing Strategy Partnership in developing
observing systems for the oceans and terrestrial carbon sources and sinks in
the global carbon cycle and in promoting systematic observations.
There is also a range of areas where present day biogeochemistry modelling is not only in need of additional data, but is also crucially limited by insufficient understanding at the level of physical or biological processes. Clarifying these processes and their controls is central to a better understanding of the global carbon cycle.
Other reports in this collection